1. Field of the Invention
The present invention generally relates to stepping motor control units and image forming apparatuses, and more particularly to a stepping motor control unit which controls a stepping motor of an equipment such as an image forming apparatus, and to an image forming apparatus which uses such a stepping motor control unit. The present invention is particularly suited for controlling the speed and the like of a stepping motor which accurately transports a recording medium such as paper in printers, facsimile machines and copying machines.
2. Description of the Related Art
FIG. 1 is a system block diagram generally showing a printer control system. The printer control system shown in FIG. 1 includes an image data storage part 61 which stores image data, an image data processing controller 62, a printer control unit 63, a driving system equipment 64 such as a stepping motor, a heating system equipment 65 such as a heat roller fixing unit, an optical system equipment 66 such as a laser head, other equipments 67, and sensors 68 which are required to control the various equipments. For example, the image data stored in the image data storage part 61 are obtained by an input from a personal computer, a facsimile signal, an input from an image scanning or the like.
The image processing controller 62 obtains the image data from the image data storage part 61, and converts the image data into a format which can be understood by the printer control unit 63 before supplying the image data to the printer control unit 63. The printer control unit 63 controls the equipments 64 through 67 in order to print the image data supplied thereto. The printer control unit 63 reads outputs of the sensors 68 during the print control process, and switches the control operation.
FIG. 2 is a diagram generally showing an example of a printer. The printer shown in FIG. 2 includes a resist motor 70, a resist roller 71 which is driven by the resist motor 70 and transports paper, a pickup roller 72 which picks up the paper from a paper cassette 79, an eject roller 73, a transport path 74, a photo sensor 75 for detecting the position of the paper, a photoconductive drum 76, a laser optical system unit 77 for forming a latent image on the photoconductive drum 76 by writing an optical image, a heat roller fixing unit 78 for fixing a toner image on the paper, and the paper cassette 79. The illustration of a developing part and a fixing part of the printer is omitted in FIG. 2.
The resist roller 71, the pickup roller 72 and the eject roller 73 shown in FIG. 2 are driven by a stepping motor. A description will be given of a speed control of the resist motor 70 which drives the resist roller 71. When a print start instruction is input, the pickup roller 72 rotates and picks up the paper from the paper cassette 79, and in addition, the resist motor 70 is rotated to transport the paper. The resist motor 70 is accelerated to a predetermined speed and thereafter assumes a constant speed state by maintaining the predetermined speed. For example, when the resist motor 70 is in the constant speed state and the trailing edge of the paper passes the photo sensor 75, an output of the photo sensor 75 changes from an ON state to an OFF state, that is, falls to a low level, thereby decelerating the resist motor 70.
FIG. 3 is a system block diagram showing a conventional stepping motor control unit of the printer. The stepping motor control unit shown in FIG. 3 includes a processor (CPU) 1, a timer resistor and down counter 2, a mutual excitation waveform output circuit 3, a motor driving circuit 4, a stepping motor 5, an interrupt generating circuit 6, an input port 7 for inputting a paper position state information signal from a sensor, and a ROM 8 which stores motor speed control data.
The processor 1 is located within the printer control unit 63 shown in FIG. 1. The processor 1 writes a time value in the timer register and down counter 2, and also control other parts of the printer. These other parts of the printer include the driving system equipment 64, the heating system equipment 65, the optical system equipment 66, and the other equipments 67. The timer register and down counter 2 outputs a time completion pulse when the time value instructed by the processor 1 is measured. This time completion pulse is input to the mutual excitation waveform output circuit 3. When the time completion pulse is input, the mutual excitation waveform output circuit 3 switches the mutual excitation waveform. An output of the mutual excitation waveform output circuit 3 is subjected to a power amplification in the motor driving circuit 3 before being applied to the stepping motor 5.
The time completion pulse is also input to the interrupt generating circuit 6. The interrupt generating circuit 6 turns ON an interrupt signal S when the time completion pulse is input. When the interrupt signal S is turned ON, the processor 1 executes a processing program for controlling the stepping motor 5. The processing program for controlling the stepping motor 5 controls the stepping motor 5 by referring to the data stored in the ROM 8 and the sensor output received via the input port 2.
The ROM 8 includes an acceleration table 8a and a deceleration table 8b. In the acceleration table 8a, "phase-A and 10 .mu.s" are written at an address 0 as shown in FIG. 4, indicating that the phase-A is to be excited first and that the time value is 10 .mu.s. In addition, in the acceleration table 8a, "8 .mu.s" is written at an address 1, "6 .mu.s" is written at an address 2, and "4 .mu.s" is written at an address 3. In addition, "2 .mu.sF" is written at an address 4 to indicate that the time value is 2 .mu.s and this data is the last data in the acceleration table 8a.
On the other hand, in the deceleration table 8b, "4 .mu.s" is written at an address 0, "6 .mu.s" is written at an address 1, and "8 .mu.s" is written at an address 2. Further, "10 .mu.sF" is written at an address 3 to indicate that the time value is 10 .mu.s and that this data is the last data in the deceleration table 8b.
FIGS. 5 and 6 are diagrams for explaining example of an acceleration control, a constant speed control, and a deceleration control of the stepping motor 5. FIG. 5 shows excitation pulses during each period, and FIG. 6 shows a mutual excitation waveform for each phase during a start control and a constant speed control of a 2-phase excitation type stepping motor. In FIG. 6, an interrupt signal SS shown below the interrupt signal S is obtained by the present invention, as will be described later.
FIG. 7 is a diagram for explaining signal exchanges between the timer register down counter 2 and the mutual excitation waveform output circuit 3. The time value is written in a timer register of the timer register and down counter 2 by the processor 1. When the processor 1 outputs a timer start trigger signal, the value in the timer register of the timer register and down counter 2 is loaded into a down counter of the timer register and down counter 2, and the value of the down counter is decremented by 1 every time a time clock is input thereto. After outputting the timer start trigger signal, the processor 1 writes the next time value in the timer register of the timer register and down counter 2. The processor 1 supplies an output enable signal to the mutual excitation waveform output circuit 3 at the same time as supplying the timer start trigger signal to the timer register and down counter 2. As a result, the mutual excitation waveform output circuit 3 excites a phase specified by the processor 1, such as a phase .phi.A, for example.
When the down counter of the timer register and down counter 2 outputs a time completion pulse, the value of the timer register of the timer register and down counter 2 is loaded into the down counter of the timer register and down counter 2, and the down counter again starts to measure the time. The time completion pulse is supplied to the mutual excitation waveform output circuit 3 as a mutual excitation count-up pulse. In response to the mutual excitation count-up pulse, the mutual excitation waveform output circuit 3 switches the mutual excitation waveform, and excites the phases .phi.A and .phi.B, for example.
FIG. 8 is a diagram showing the construction of the mutual excitation waveform output circuit 3 shown in FIG. 3. The mutual excitation waveform output circuit 3 shown in FIG. 8 includes a mutual excitation waveform register 110, a 3-bit up-down counter 111, and a waveform decoder 112.
The processor 1 writes data into the mutual excitation waveform register 110. The contents of the mutual excitation waveform register 110 are loaded into the 3-bit up-down counter 111. The 3-bit up-down counter 111 operates as an up-counter when the stepping motor 5 rotates in a forward direction, and operates as a down-counter when the stepping motor 5 rotates in a reverse direction. The value of the 3-bit up-down counter 111 is incremented by 1 or is decremented by 1 in response to the mutual excitation count-up pulse. The waveform decoder 112 decodes the value of the 3-bit up-down counter 111 in response to an output enable signal which is ON, and excites a phase which is determined by the decoded result. For example, the waveform 112 excites the phase .phi.A when the value of the 3-bit up-down counter 111 is "000", and excites the phases .phi.A and .phi.B when the value of the 3-bit up-down counter 111 is "001".
Next, a description will be given of the operation of the stepping motor control unit shown in FIG. 3. When rotating the stepping motor 5, the processor 1 sets the time value (10 .mu.s) at the address 0 of the acceleration table 8a into the timer register of the timer register and down counter 2, and sets the phase-A into the mutual excitation waveform register 110. In addition the processor 1 supplies the timer start trigger signal to the timer register and down counter 2, and supplies the output enable signal to the mutual excitation waveform output circuit 3. After supplying the timer start trigger signal to the timer register and down counter 2, the processor 1 sets the time value (8 .mu.s) at the address 1 of the acceleration table 8a into the timer register of the timer register and down counter 2.
When the timer register and down counter 2 measures the time (initially 10 .mu.s) instructed by the processor 1, the time completion pulse is output, and in addition, the time value (8 .mu.s in this case) of the timer register is loaded into the down counter of the timer register and down counter 2. When the time completion pulse is output, the mutual excitation waveform output circuit 3 switches the mutual excitation waveform, and the interrupt generating circuit 6 turns ON an interrupt signal S.
When the interrupt signal S is turned ON, the processor 1 executes a processing program for controlling the stepping motor 5. This stepping motor control program carries out a process by referring to the acceleration table 8a during acceleration of the stepping motor 5, carries out a process based on the sensor output during constant speed rotation of the stepping motor 5, and carries out a process by referring to the deceleration table 8b during deceleration of the stepping motor 5.
In this particular case where the stepping motor 1 is accelerating at the present, the processor 1 reads the data (6 .mu.s) at the address 2 of the acceleration table 8a when the interrupt signal S is turned ON, and sets this data into the timer register of the timer register and down counter 2. Thereafter, the processor 1 carries out a similar operation every time the interrupt signal S is turned ON. When the processor 1 reads the data (2 .mu.s) at the address 4 of the acceleration table 8a, the processor 1 recognizes that this data is the last data in the acceleration table 8a, turns ON a constant speed rotation flag, and sets the time value (2 .mu.s) into the timer register of the timer register and down counter 2. The mutual excitation waveform is thereafter switched for every 2 .mu.s.
When the interrupt signal S is turned ON during the constant speed rotation of the stepping motor 5, the processor 1 checks whether or not the sensor output made a transition from an ON state (indicating the existence of the paper) to the OFF state. If the sensor output changes from the ON state to the OFF state, the processor 1 turns ON a deceleration flag. On the other hand, the processor 1 does not carries out a specific process if the sensor output does not change.
When the interrupt signal S is turned ON in a state where the deceleration flag is turned ON, the processor 1 sets the data (4 .mu.s) at the address 0 of the deceleration table 8b into the timer register of the timer register and down counter 2. Next time when the interrupt signal S is turned ON, the processor 1 reads the data (6 .mu.s) at the address 1 of the deceleration table 8b, and sets the read data into the timer register of the timer register and down counter 2. A similar process is thereafter carried out every time the interrupt signal S is turned ON.
According to the conventional stepping motor control unit, the processor constantly reads the output of the paper position detecting sensor, and the output of the paper position detecting sensor is read every time the mutual excitation waveform is switched even during the constant speed rotation of the stepping motor when the speed control is unnecessary. For this reason, the ratio of the execution time of the processing program for controlling the stepping motor becomes large with respect to the total time in which the processor is operating, and there was a problem in that the control of the processor with respect to the other equipments became limited thereby,